1. Field of the Invention
The present invention is directed to the drying of surfaces after fluid treatment or wet processing. More particularly, the invention relates to the manufacture of semiconductor components, and in particular to the preparation of semiconductor wafers prior to high temperature processing steps, such as diffusion, ion implantation, epitaxial growth, and chemical vapor deposition steps. Still more particularly, the invention relates to methods and apparatus for the drying of semiconductor wafers after prediffusion cleaning.
2. Prior Art
In the fabrication of semiconductor wafers several process steps require contacting the wafers with fluids. Examples of such process steps include etching, photoresist stripping, and prediffusion cleaning. The equipment conventionally used for contacting semiconductor wafers generally consists of a series of tanks or sinks into which racks of semiconductor wafers are dipped. Wafer carriers are described, for example, in U.S. Pat. Nos. 3,607,478, 3,964,957 and 3,977,926. Such conventional wet processing apparatus poses several difficulties.
Since the tanks are open to the atmosphere, airborne particulates can enter into the process solutions. Through surface tension these particles are easily transferred to the wafer surfaces as the wafers are dipped into and lifted out of the sinks. This particulate contamination is extremely detrimental to the microscopic circuits which the wafer fabrication process creates. It is especially important to minimize particular contamination during prediffusion cleaning.
After fluid processing the wafers normally need to be dried. This can be a particularly challenging process because it is important that no contamination be created during the drying process. Evaporation is undesirable since it often leads to spotting or streaking. Even the evaporation of ultra high purity water can lead to problems because such water is very aggressive to the water surface and will dissolve traces of silicon and silicon dioxide during even short periods of water contact. Subsequent evaporation will leave residues of the solute material on the wafer surface. Contamination and other causes of semiconductor failure are discussed, for example, in J. Schadel, "Device Failure Mechanisms In Integrated Circuits," Solid State Devices 1983 Conf. Ser. No. 69 (Institute of Physics, London 1984) 105-120.
Conventionally, semiconductors are dried through centrifugal force in a spin-rinser-drier. Because these devices rely on centrifugal force to "throw" water off the wafer surfaces, their use results in several problems. First, there are mechanical stresses placed on the wafers which may result in wafer breakage, particularly with larger wafer sizes. Second, because there are many moving parts inside a spin-rinser-drier, contamination control becomes a difficult problem. Third, since the wafers conventionally travel at high velocity through dry nitrogen, static electric charges develop on the wafer surfaces. Since oppositely charged airborne particles are quickly drawn to the wafer surfaces when the spin-rinser-drier is opened, particular contamination results. Fourth, it is difficult to avoid evaporation of water from the surfaces of the wafers during the spin process with the attendant disadvantages discussed above.
More recently, methods and apparatus have been developed for steam or chemical drying of wafers, including the method and apparatus disclosed in our U.S. Pat. No. 4,778,532. Chemical drying generally comprises two steps. First, the rinsing fluid, preferably water is driven off the wafers and replaced by a nonaqueous drying fluid. Second, the nonaqueous drying fluid is evaporated using a predried gas, preferably an inert gas such as nitrogen at a low flow velocity.
Another chemical drying process currently used in Japan consists of sequentially immersing the wafer carrying vessel in tanks of deionized water, followed by suspending the wafers above a tank of boiling isopropanol. The wafer-carrying vessel is then slowly withdrawn from the isopropanol vapor to pull the water droplets off the wafer surfaces.
The most important feature for an effective wafer drying technology is that the wafers produced by ultraclean, i.e., with minimum particle contamination and minimum chemical residue. Because many drying solvents are flammable, safety is also a very important consideration. Other important design criteria include low chemical consumption, low waste generation, and automated handling with little or no operator exposure.